Single transverse mode light sources are important in communication system applications. By providing a fundamental or zero-order transverse mode optical signal, the light source is more efficiently coupled to the optical transmission medium and to other optical system components. In laser machining and micrographics system applications, a fundamental transverse mode light source exhibits an energy distribution in which the energy is concentrated along the optic axis of the laser resonator.
Heterostructure semiconductor lasers have been used for obtaining fundamental transverse and longitudinal mode optical signals. In heterostructure semiconductor lasers, transverse mode control is usually achieved by narrowing the active region of the laser through the use of growth techniques or etching and regrowth techniques or special laser geometries. While the active region may be narrowed to achieve fundamental transverse mode operation for the laser, the volume of the pumped optical cavity is similarly reduced causing a corresponding reduction of the available optical output power from the laser.
In U.S. Pat. No. 3,790,902, a semiconductor heterostructure gain medium has been coupled to a passive dielectric waveguide to form a monolithic laser structure. This structure produces a single transverse mode optical output signal provided that the cross-section of the guiding portion in the passive dielectric waveguide is congruent with the cross-section of the active region in the gain medium, both cross-sections being measured perpendicular to the optic axis of the laser. Additional constraints are placed on the length of the dielectric waveguide, the refractive indices of the dielectric waveguide, and the refractive index difference between the active region and the dielectric waveguide. In addition to these design constraints, the laser does not afford ready coupling to transmission media such as optical fibers. Moreover, fabrication of the laser involves complicated processing steps to produce an aligned, monolithic device.